The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is directed to a top nozzle subassembly having hold-down means incorporated therewithin and which is removably mounted on the upper end portions of its control rod guide thimbles.
In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements or rods grouped in and supported by frame works referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower transversely extending core support plates. These upper and lower core support plates are directly or indirectly attached to a support barrel which surrounds the entire core and extends between the ends thereof. In the most common configuration, the axis of the core support barrel extends vertically and the various fuel assemblies are also arranged vertically resting on the lower support plate. To facilitate handling and installation, the fuel assemblies are generally not secured to the lower core support plate.
Temperatures at various times within the core may vary greatly, such as, from cold shutdown to normal operating conditions. It is also a well known fact that different materials exhibit different thermal growth characteristics. Therefore, since the materials used in the vertically extending support structures of the fuel assemblies are generally different than those used in the core support barrel, the thermal expansion of these various members in the axial or vertical direction may be quite significant, particularly, at the high temperatures found within the core and the axial length of some of the members. For these reasons, the fuel assemblies are not usually attached to the upper and lower core plates but rather are supported in a manner which permits some relative motion therebetween. The axial thermal expansion differential between the fuel assemblies and the core support barrel has been accommodated by insuring that the axial spacing between the upper and lower core support plates is somewhat greater than the axial length of the fuel assemblies. Normally, this is accomplished by providing an axial gap between the top of the fuel assemblies and the upper core support plate. Over the years, the axial gap spacing had to be increased due to the increasing temperatures in the core region, the increased length and different construction of the fuel assemblies, as well as from the use of different materials, such as Zircaloy. This axial gap spacing not only requires critical design tolerances and precise positioning of the upper core plate over the fuel assemblies, but also, as felt by many designers, permits crossflow of the upwardly flowing coolant in this upper region, subjecting some of the core elements to a potentially damaging sideload.
Generally, in most reactors, a fluid coolant such as water, is directed upwardly through apertures in the lower core support plate and along the fuel rods of the various fuel assemblies to receive the thermal energy therefrom. The physical configuration of the fuel assemblies is such that the coolant may experience a significant pressure drop in passing upwardly through the core region. This pressure drop necessarily produces a lifting force on the fuel assemblies. In some instances, the weight of the fuel assembly is sufficient to overcome the upward hydraulic lifting forces under all operating conditions; however, this is often not the case, particularly when the coolant density is high, as at reactor startup, and additionally because of increasing coolant flow rates. When the hydraulic forces in the upward direction on a particular fuel assembly are greater than the weight of that fuel assembly, the net resultant force on the fuel assembly will be in the upward direction, causing the assembly to move upward into contact with the upper core plate. This upward motion of the fuel assembly, if uncontrolled, may result in damage to the fuel assembly and the fuel rods or to the upper core plate and must, therefore, be avoided. In order to prevent hydraulic lifting of the fuel assemblies, various hold-down devices have been developed.
One such hold-down device, as seen in U.S. Pat. No. 3,379,619, employs the use of leaf springs. The leaf springs are disposed in the axial gap, between the top of the fuel assembly and the upper core plate, which has been provided to accommodate for the thermal expansion of the fuel assembly. More particularly, the leaf springs are attached to the top flange of an enclosure structure having upstanding sidewalls and a bottom adapter plate, with the adapter plate being attached to the upper ends of the control rod guide thimbles. The leaf springs are held in a state of compression within the axial gap and cooperate with the upper core plate to prevent the fuel assembly from being moved upwardly, by the hydraulic lifting forces of the coolant, into damaging contact with the core plate, while, at the same time, allowing for thermal expansion of the fuel assembly into the axial gap. The integrally formed top flange on the enclosure not only provides a physical location for mounting the leaf springs, but also, provides a surface for alignment holes that interface with pins projecting down from the upper core plate. In addition, the enclosure: provides convenient means for physically handling the fuel assembly during installation and removal; protects the fuel assembly from side loading; and, one of its primary purposes, channels the fluid coolant upwardly to prevent crossflow at the top portion of the fuel assembly. Although, the leaf spring hold-down device has many advantages, it necessitates the requirement of an axial gap to provide for thermal expansion, and further, requires physical space for mounting the leaf springs. In some fuel assemblies, especially in some of the more newer designs with different fuel rod arrangements and configurations, there is not sufficient physical space to mount the leaf springs.
Another type of hold-down device, such as the one shown in the Klumb et al patent (U.S. Pat. No. 3,770,583), employs the use of coil springs. The device basically includes coil springs disposed about upright alignment posts having one end threadably secured to the top end plate of a fuel assembly, which in turn, is mounted on the upper ends of the control rod guide tubes. A hold-down plate is slidably mounted on the alignment posts and the coil springs are interposed between the two plates. The upper ends of the alignment posts are radially enlarged to form shoulders for retaining the hold-down plate on the posts. In use, the coil springs bias the hold-down plate upwardly against a core alignment plate to provide a downward force on the fuel assembly. To accommodate for thermal expansion of the fuel assembly, it is mandatory that aligned clearance holes be provided in the upper core plate for upward movement of the enlarged shoulders of the alignment posts. Machining of such clearance holes is not only costly, but also weakens the upper core plate structure. Furthermore, the arrangement lacks an enclosure structure to prevent crossflow of the coolant in the upper region of the fuel assembly.
Two modified versions of the coil spring hold-down device seen in the Klumb et al patent, can be seen in FIGS. 3 and 3a of the Anthony patent (U.S. Pat. No. 4,192,716). The version represented in FIG. 3 is essentially the same as the Klumb et al device, with the difference being the provision of an axial gap between the top of the fuel assembly and the upper core plate. The FIG. 3a version also requires an axial gap spacing to accommodate for thermal expansion. Hydraulic lift is prevented by coil springs, seated in cavities in the upper core plate, that cooperate with an alignment pin arrangement. Both of these versions suffer from some of the same shortcomings as the Klumb et al device in addition to the disadvantages associated with an axial gap spacing.